The invention relates to a device for controlling the advancing movement of a casting plunger in a casting chamber of a cold-chamber die casting machine by means of an actuating signal. The invention is specifically concerned with the control of the advancing movement of the casting plunger during a time period referred to in the present case as the chamber filling movement phase from a partial filling position of the casting plunger, with a partially filled casting chamber starting volume, to a full filling position of the casting plunger, with a filled casting chamber remaining volume.
As is known, in cold-chamber die casting a molten material to be cast, for example a molten metal alloy substantially comprising aluminum and/or magnesium and/or zinc, is introduced into a horizontally arranged casting chamber and is subsequently transported into a casting mold by a casting plunger driven hydraulically or in some other way. This operation is performed cyclically for the purpose of the multiple production of identical products, molten material being forced into the casting mold each time in every casting cycle. Cylindrical casting chambers with a circular cross section are used almost exclusively for this. The introduction of the molten material into the casting chamber may be performed in various ways, under atmospheric pressure, under positive pressure or under negative pressure, for example by filling via a filling opening of the casting chamber by means of a casting ladle or by suction intake by means of generating a negative pressure in the casting chamber. The amount of molten material introduced into the casting chamber depends on the respective casting mold volume, i.e. the volume of the part to be cast, so that, depending on the cast part, different filling levels in the casting chamber apply and, after the introduction of the molten material, a certain volume of air lying above remains in the horizontally arranged casting chamber cylinder as long as the casting plunger is still in an initial position on a rear side, facing away from the casting mold, of the casting chamber cylinder behind a casting chamber inlet. The term volume of air in the present case also comprises generally the case where it is an upper partial volume of the casting chamber that is filled with a different gas or evacuated.
In a first phase of the advancing movement of the casting plunger, the casting plunger is moved forward from its initial position, in which, as explained, the casting chamber is partially filled, to the full filling position, in which the casting chamber volume successively reduced by the advancing movement of the casting plunger is just completely filled with the filled molten material. This is followed by the injection operation (which is of no further interest in the present case), by which the molten material is forced out of the casting chamber via a casting chamber outlet, facing the casting mold, on a front side of the casting chamber cylinder and the adjoining runner, as it is known, into the casting mold. During the initial chamber filling movement phase, there is the problem of undesired air/gas inclusions in the molten material if the plunger advancing movement progresses unfavorably. Such air/gas inclusions in the molten material may lead to increased porosity and, depending on the use or further processing of the cast part, consequently to unsatisfactory quality of the cast part.
Two effects are responsible for this in particular, as depicted in FIG. 1 and FIG. 2, for purposes of illustration in three part-images respectively, with a casting plunger 2 successively advanced in a horizontally arranged casting chamber cylinder 1, the casting chamber 1 initially being partially filled with a molten material 3, as shown by the respectively uppermost part-image, and the casting plunger 2 being located on a rear side 1a, facing away from the casting mold, of the casting chamber 1 behind a casting chamber inlet 4. FIG. 1 shows the creation of a wave breaker 5, i.e. a breaking wave, of the molten material 3 forced forward by the casting plunger 2 in the casting chamber 1, i.e. in the direction of a front side 1b, facing the casting mold, of the casting chamber 1. FIG. 2 depicts the effect of a premature brief separation of the wave from the casting plunger 2 and/or premature wave reflection at a front end 1c, facing the casting mold, of the casting chamber 1, i.e. with this unfavorable control of the plunger advancing movement a wave of molten material 6 begins to creep forward away from the plunger 2. If this wave 6 reaches the top of the casting chamber directly or else after reflection, it cuts off a volume of air/gas 7 at the casting plunger 2 from a casting chamber outlet 8 lying at the front, as shown in the lower part-image of FIG. 2. Both effects lead to increased air/gas inclusions, as schematically symbolized as small bubbles 9 in the lowermost part-image of FIG. 1 for the case of the wave breaking.
It is an object of the invention to provide a device of the type mentioned at the outset with which the advancing movement of the casting plunger can be controlled, specifically in the chamber filling movement phase, in such a way that the amount of air/gas inclusions in the molten material can be reduced or minimized, which typically leads to reduced porosity in the finished cast part.
The invention solves this problem by providing a control device in which a respective associated progression of the actuating signal is provided for different specified sets of values of a plurality of process parameters that influence the movement of the molten material in the casting chamber during the chamber filling movement phase, which progression is defined as the most suitable actuating signal progression for the particular set of parameter values. The control device is designed to use the most suitable actuating signal progression in dependence on values of the process parameters pertaining at the beginning of a casting cycle for controlling the casting plunger advancing movement during the chamber filling movement phase, the plurality of process parameters including at least one of a group of parameters, said group of parameters comprising at least one casting chamber geometry parameter, at least one filling amount parameter, at least one casting mold parameter, at least one casting chamber temperature, and at least one molten material temperature parameter.
In the control device according to the invention, a respective associated progression of an actuating signal is provided for different specified sets of values of a plurality of process parameters that influence the movement of the molten material in the casting chamber during the chamber filling movement phase, also referred to in the present case as parameters for short, and is used by said device to control the advancing movement of the casting plunger during the chamber filling movement phase from an initial partial filling position, with a partially filled casting chamber starting volume, to the full filling position, with a filled casting chamber remaining volume. The actuating signal progressions provided are in this case progressions for which it is defined that in each case one of them is the most suitable for the particular set of parameter values. “Most suitable” should be understood here as meaning that the actuating signal progression assigned to the particular set of parameter values leads to that progression of the plunger advancing movement that reduces or avoids the undesired effects mentioned, of wave breaking and of cutting off a volume of air, better in the current situation described by the particular set of parameter values than all the other progressions of the plunger advancing movement considered. Apart from this primary quality criterion, the definition as “most suitable” is of course also arrived at by taking into account customary criteria relevant to the casting process, such as the smallest possible time requirement for the casting cycle, and consequently for the plunger advancing movement. The choice of this most suitable actuating signal progression consequently allows the introduction of air/gas into the molten material, and consequently the porosity in the cast part, to be kept as low as possible for each casting cycle, without appreciably slowing down the casting cycle as compared with conventional casting process controls.
The control device according to the invention is correspondingly designed to use this most suitable actuating signal progression in dependence on values of the process parameters pertaining at the beginning of a casting cycle. For this purpose, it may preferably be provided that the possible most suitable actuating signal progressions for various specified sets of values of the parameters taken into account are determined in advance, i.e. before the running time of the casting process or casting cycle, and are stored in the control device. The control device then selects for each casting cycle the actuating signal progression most suitable for the current set of parameter values for controlling the advancing movement of the casting plunger during the chamber filling movement phase. This determination in advance of various progressions of the plunger advancing movement, i.e. different progressions of the relevant actuating signal, may be performed empirically on the actual object or preferably systematically, and consequently deterministically, on the basis of corresponding computer simulations with suitable computational models. The latter makes it possible to carry out a comparatively large number of “tests” with varying values of the relevant process parameters. If the simulation is carried out before the running time of the casting process, the computing time is not restricted to the typical duration of a casting cycle, which allows the use of a relatively computationally intensive model that describes the flow conditions of the molten material in the casting chamber during the plunger advancing movement comparatively well. The simulated model system may also be in particular a simulated closed-loop control system with a closed-loop controller, which attempts to correct computationally established deviations from a desired molten material flow characteristic by corresponding controller interventions. In this way, the most suitable actuating signal progression for the respective starting situation, as described by the currently used set of parameter values, can be determined very accurately by means of model-aided closed-loop control simulation. Alternatively, a direct determination of the actuating signal progression provided may be provided during the running time of the casting process.
The plurality of process parameters influencing the movement of the molten material in the casting chamber during the chamber filling movement phase comprise at least one parameter concerning the casting chamber geometry, at least one parameter concerning the filling amount of molten material in the casting chamber, at least one parameter concerning the casting mold and/or at least one parameter concerning the temperature of the casting chamber and/or the molten material. It is found that, by taking one or more of these parameters into account, it is already possible to obtain usable actuating signal progressions for the plunger advancing movement that to the greatest extent avoid the undesired effects with respect to wave breaking or premature wave separation/wave reflection. Depending on the application, one or more further parameters may be taken into account. Each parameter should be understood here as meaning that, depending on the application, it may comprise current values and/or values originating from one or more previous casting cycles and/or values determined from such values in combination, it being possible in each case for these to be values obtained by measuring instruments or computationally.
In a development of the invention, the plurality of process parameters comprise more specifically at least one casting chamber length parameter, at least one casting chamber height parameter, at least one casting chamber filling degree parameter, at least one molten material temperature parameter, at least one casting chamber temperature parameter and/or at least one molten material viscosity parameter and, depending on the application, optionally one or more further parameters. The geometry parameters describe the spatial boundary conditions for the movement of the molten material in the casting chamber, the temperature/viscosity parameters describe the flow behavior of the molten material and possibly also any outer layer problems, such as that known as skin hardening of the molten material on the casting chamber inner wall.
In an advantageous development of the invention, the actuating signal progressions provided are grouped into a plurality of types with a differing number of successive stages of the progression, each stage representing an associated rise in the height of the molten material at the casting plunger. It is found here that, for example depending on the filling amount of molten material, and consequently the degree of filling of the casting chamber, a single-stage or multi-stage actuating signal progression is favorable, each stage comprising initially raising the filling level of the molten material at the plunger more rapidly by a specifiable degree and then keeping it substantially constant, or at most changing it more slowly. The grouping of all the possible actuating signal progressions in a discrete set of progressions with a differing number of stages also has advantages with regard to the memory space requirement for storing most suitable actuating signal progressions determined in advance, with regard to rapid access to the stored data for the selection of the respectively most suitable actuating signal progression and with regard to the correspondingly staged advancing velocity of the casting plunger.
In a further refinement of this aspect of the invention, each stage of the progression is defined such that it specifies an initially accelerated casting plunger movement followed by a casting plunger movement with a velocity progression that is determined from a progression determined in advance for a height of the molten material at the casting plunger. Typically, this further progression determined in advance for the height of the molten material at the casting plunger comprises that, after it has been raised relatively rapidly to a higher level by the initial accelerated plunger advancing movement, the height of the molten material is subsequently kept substantially at this new level, or at most is raised further significantly more slowly. It is found that this linking of the plunger advancing movement to a specific progression over time of the height of the molten material at the casting plunger can lead to very good most suitable actuating signal progressions for the plunger advancing movement. Moreover, this offers the optional possibility of also intervening in a controlling manner in the operation of the plunger advancing movement by continuously establishing the height of the molten material at the casting plunger by means of sensors.
In a development of the invention, the actuating signal progressions provided are obtained by a model-aided closed-loop control simulation system before or alternatively during a running time of the advancing movement of the casting plunger, with the advantages indicated above in this respect. A determination in advance allows the use of greater computer capacities, and consequently more accurate computational models. An alternative determination directly at the running time allows any current disturbing influences there may be still to be taken into account during the respective casting cycle.
In a further refinement of this aspect of the invention, the model-aided simulation closed-loop control system is integrated in the control device. As a result, it is located at the place of use of the control device, i.e. typically at the location of the associated casting machine, which is favorable in particular for the cases where a determination of the most suitable actuating signal progression directly at the running time of the casting process is provided, or it is intended to enable the casting machine user to determine most suitable actuating signal progressions itself in advance by model-aided closed-loop control simulation for the particular casting machine system.